The gate stack has historically been manufactured from polysilicon on silicon dioxide or nitrided silicon oxide (SiON). Recently, the polysilicon electrode material in the stack has been replaced with metal and the silicon dioxide or SiON dielectric material in the stack has been replaced with a high-k material to improve transistor performance. For example, the metal gate electrode, being a good electrical conductor, suppresses the gate depletion layer of a MOS transistor. This arrangement changes the gate electrode/gate dielectric interface to metal on a high-k dielectric.
To further complicate the situation, to optimize performance, different metal-comprising gate materials are generally needed to provide a different work function for the NMOS devices versus the PMOS devices (generally referred to as a dual work function metal gate). In some gate stack arrangements for the dual work function metal gate, one of the metal-comprising gate electrode layers (e.g., for PMOS devices) comprises a readily oxidizable metal, such as tungsten (W).
The metal gate etch process is generally followed by a cleaning process (or “clean”). The clean is used to strip the photoresist pattern, and remove the post etch residues while attempting to maintain selectivity to the exposed metal. A significant yet to be met challenge for this clean is to be able to strip the photoresist without significantly oxidizing the metal gate electrode layer, and without generating a residue that is impossible or at least difficult to remove.